1. Field of the Invention
This application concerns a process and apparatus for the reclamation of polystyrene-type waste materials and a reclaimed polystyrene product. Due to the choice of dissolution solvent and the use of lower temperatures in the recovery section, the reclaimed polystyrene product will not be subject to degradation. Accordingly, the reclaimed polystyrene product will be within the specification ranges of the input polystyrene-type material.
The process and apparatus include dissolution of the waste polystyrene-type materials in a dissolve section utilizing a reusable solvent having a low boiling point and high vaporization rate, removal of solid contaminants in one or more filter sections, devolatilization of the dissolved polystyrene and recovery of the polystyrene-type material in a solid form in a recovery section. Preferably the process and apparatus include recycling and reuse in the process of the vaporized solvent from the recovery section. The process and apparatus may also include reduction of the polystyrene-type material to be reclaimed in a reduction section prior to its entry into the dissolve section. The maximum temperature in the recovery section is 190xc2x0 C. The reusable solvent is preferably environmentally safe and has a low boiling point and high vaporization rate. n-Propyl bromide, or environmentally safe mixtures thereof, is the preferred reusable solvent.
The recovery of plastic materials has been a focus of many organizations, governments, and individuals for some time. Some plastic materials, such as polyethylene-terephthalate and polyethylene, have found extensive use in the recycling industry. However, recycling of polystyrene-type resins has not met with the same technical or economical success, and the disposal of polystyrene-type materials continues to present unresolved environmental problems. No simple means for recovery of polystyrene-type material is presently available, especially when the polystyrene-type material are fabricated into foam material. Polystyrene foam is much less dense than non-foamed polystyrene because it contains considerable volume of air encapsulated by the solid polystyrene construction. This increases the storage, transportation and disposal costs, including the use of mechanical and chemical means of reducing the size of such waste material. Waste toner, for example, as used in copying and printing machines, is another polystyrene based product which is in need of an environmentally safe means of recovery.
Due to the lack of efficient and environmentally safe recycling means, most of the products made of polystyrene-type materials are disposed of in landfills or incinerated. However, the polystyrene resin in such landfills does not decompose and may be dissolved by leachate from decomposition of other organic matter. The resulting leachate material contaminates the methane gas generated in landfills, which is finding use as a fuel source by energy generating companies. Thus, the removal of polystyrene-type materials from solid landfill materials is desirable even though their presence could add to the fuel value. Similarly, when the polystyrene resin is incinerated, the resulting gases are toxic and heavy carbon deposits tend to cause clogging of the stacks.
Current methods for recycling polystyrene have the additional drawback of degrading the material so that it is not reusable for the same grade or quality of polystyrene-type product as the waste material. This further devalues the recovered polystyrene-type material. The present process, on the other hand, will provide a reclaimed polystyrene product within the specification ranges of the input polystyrene-type material, where the input material comprised of a single polystyrene or closely related polystyrenes. Even when reclaimed from diverse waste polystyrenes, the reclaimed polystyrene product of the invention exhibits a surprisingly high quality when compared to a similar product made from virgin polystyrene.
2. Prior Art
U.S. Pat. No. 4,517,312, to Kumasaka et al., describes a process for regenerating a resin based on dissolving the waste plastic in an organic solvent and the solution mixed with an immiscible solvent, scarcely dissolving the resin, which will then precipitate the resin. Organic solvents having specific gravities larger than water, such as methylene chloride, trichloroethylene or carbon tetrachloride are preferably used. In the examples wherein waste foam polystyrene foam is recovered, it is dissolved in methylene chloride and then mixed with water resulting in the resin separating out at the interface of the liquids. Such solvents are not considered environmentally safe.
U.S. Pat. No. 5,198,471, to Nauman et al., describes a process for selectively separating various plastic materials found in a typical waste stream from a household. The process utilizes specific solvents to separate each type of plastic of the commingled stream, then separates the resulting solution from the remaining materials and removes the solvent to recover the specific resin. Tetrahyrofuran, toluene, and xylene are used to dissolve mixtures of polystyrene (PS) materials with other plastic materials, resulting in a mixture of PS and one or more other plastic materials in solution. The other plastic materials may be present individually or in combination and include polyvinyl chloride (PVC), low and high density polyethylene (LDPE and HDPE), polypropylene (PP), and polyethylene terephthalate (PET). However, for efficient operation of the process, the dissolved solids concentration must be either very little  less than 20% or concentrated to  greater than 80%, preferably  less than 10% or concentrate to  greater than 80%, to be effective. For a PVC/PS mixture first dissolved in tetrahydrofuran, methylene chloride may be used to separate out the PS. The flash evaporation (devolatilization) temperature is between 200xc2x0 C. and 400xc2x0 C. and takes the solution to a concentration of 50-95% by weight of polymer solids.
U.S. Pat. No. 5,223,543, to Iovino, describes a process for reducing the volume of polystyrene foam using d-limonene as a solvent.
U.S. Pat. No. 5,269,948, to Krutchen, describes decontamination of polystyrene using styrene monomer to dissolve the contaminated polystyrene. The resultant solution is then used in the polymerization of polystyrene.
U.S. Pat. No. 5,300,267, to Moore, describes process and apparatus to reclaim solid polystyrene type waste materials. It includes a dissolve unit and a recovery unit that is based on thin film evaporation and higher temperatures for processing with the solvent, the specific solvent being perchloroethylene.
U.S. Pat. No. 5,629,352, to Shiino et al., describes a process for reduction of polystyrene foam and the recycling of such using solvents consisting of a mixture of a glycol ether and dialkyl ester. The solvent comprises at least one member selected from the group consisting of such a glycol ether compound as diethylene glycol dimethyl ether, diethylene glycol diethyl ether, or dipropylene glycol dimethyl ether and such a fatty acid dialkyl ester compound as dimethyl succinate, dimethyl glutarate or dimethyl adipate. These solvents have flash points of 100xc2x0 C. or greater and need the addition of water to be made non-flammable.
U.S. Pat. No. 5,824,709, to Suka, discloses a process wherein a waste plastic containing a polymer such as polystyrene, polypropylene or ABS resin is dissolved in a solvent to forma solution of waste plastic at elevated temperature. The solution is filtered to remove labels or adhered foreign debris. The filtrate is heated to a temperature of 200xc2x0 C. to 300xc2x0 C. under a pressure of 1-75 torrs to remove the solvent by evaporation, and the resulting plastic melt is extruded into pellet whereby the waste plastic is recovered. If the plastic material to be recovered is polystyrene, toluene and ethylbenzene are the most preferred solvents.
U.S. Pat. No. 5,891,403, to Badger et al., describes a portable waste disposal unit for treating waste polystyrene, especially foamed polystyrene, using perchloroethylene as the solvent. The waste polystyrene solution is later shipped to an appropriate waste recovery facility.
The present invention provides apparatus, operatively interconnected, to reclaim polystyrene-type polymer waste material, comprising an optional reduction section, a dissolve section, one or more filtering sections, and a recovery section to recover the polystyrene-type material in solid form, and preferably includes means to remove, store and reuse the solvent in the apparatus. The starting material may be foamed or non-foamed solid polystyrene materials, preferably substantially free of other types of plastic materials. The present invention also provides a process for reclaiming waste polystyrene-type plastic materials, said process comprising the steps of (1) optionally reducing the size of the bulk polystyrene-type material in a reduction unit and transferring the size-reduced polystyrene-type material to a dissolve unit, (2) dissolving the polystyrene material in a reusable polystyrene solvent in the dissolve unit, (3) transferring the solution of polystyrene-type material to a preheater unit, (4) superheating the solution of polystyrene-type material to a temperature of less than 190xc2x0 C. in the preheater unit, (5) transferring the superheated solution of polystyrene-type material to an extruder (6) devolatilizing the solution of polystyrene-type material at different points as it moves through the extruder while maintaining the temperature at less than 190xc2x0 C., and (7) cooling the extrudate to form solid reclaimed polystyrene-type material. Preferably, the reusable polystyrene solvent is removed and transferred to a storage unit or directly to the dissolve section during the recovery stage.
This process provides reclaimed waste polystyrene-type plastic materials for reuse within the specification ranges of the input polystyrene-type material, particularly when the waste polystyrene is comprised of a single polystyrene or closely related polystyrenes. Where the reclamation is from diverse waste polystyrenes, the reclaimed polystyrene product of the invention exhibits a surprisingly high quality when compared to a similar product made from virgin polystyrene.
The subject invention provides a process for reclaiming waste polystyrene-type plastic materials, said process comprising the steps of
(1) dissolving the polystyrene material in a reusable polystyrene solvent in a dissolve unit,
(2) transferring the solution of polystyrene-type material to a preheater unit,
(3) superheating the solution of polystyrene-type material to a temperature of less than 190xc2x0 C. in the preheater unit,
(4) transferring the superheated solution of polystyrene-type material to an extruder,
(5) devolatilizing the solution of polystyrene-type material at different points as it moves through the extruder while maintaining its temperature at less than 190xc2x0 C., and
(6) recovering the extrudate reclaimed polystyrene-type material in solid form. Optionally, the process can provide a first step which comprises reducing the size of the bulk polystyrene-type material in a reduction unit and transferring the size-reduced polystyrene-type material to the dissolve unit.
The reusable polystyrene solvent preferably has a low boiling point, preferably 35xc2x0 C. to 90xc2x0 C. and a high evaporation rate, preferably 3-7 [AST D353976, butyl acetate=1], allowing devolatilization of the semisolid extrudate to occur at lower temperatures (i.e., less than 190xc2x0 C.) than is standard for the production of polymer polystyrene and other processes that utilize solvents for waste polystyrene recycling. Additionally, the reusable polystyrene solvent is preferably environmentally safe. n-Propyl bromide, or an environmentally safe mixture thereof having a low boiling point and high vaporization rate as above, is a preferred reusable solvent. Environmentally safe means that the solvent is non-flammable and non-hazardous, and preferably non-corrosive, as determined by US OSHA and EPA regulations. More preferably, environmentally safe also means that the solvent meets the Montreal Protocol for Global Warming Potential (GWP) and Ozone Depletion Potential (ODP) and having a US Postal Service designation of non-hazardous. Preferably the reusable solvent may itself be reclaimed, that is treated to return it substantially to its original components.
n-Propyl bromide may be obtained commercially as Ensolv(copyright) from EnviroTech International, Inc., Melrose Park, Ill., U.S.A. The General Use n-propyl bromide product is preferred. These n-propyl bromide products are further described in U.S. Pat. Nos. 5,616,549 and 5,824,162, both to Lawrence A. Clark, which are incorporated herein by reference in their entirety. Generally, they are composed of 90 to 96.5% n-propyl bromide and 0% to 6.5% of a mixture of terpenes and 3.5% to 5% of a mixture of low boiling solvents. EnSolv(copyright) is environmentally safe, including meeting the standards of the Montreal Protocol for Global Warming Potential (GWP) and Ozone Depletion Potential (ODP) and having a US Postal Service designation of non-hazardous. For use in the present invention, n-propyl bromide or such environmentally safe mixtures thereof, may further include environmentally safe diluents, such as, isopropyl alcohol. When isopropyl alcohol is used as a diluent it preferably comprises 10 per cent or less of the mixture by weight.
In the process of the invention the temperature is maintained at less than 190xc2x0 C., preferably less than 180xc2x0 C., throughout the process, preferably less than 100xc2x0 C. in the dissolve and filtering sections and less than 160xc2x0 C. in the recovery section. Also, the overall process time is preferably maintained at less than 4 hours, and more preferably at less than 2 hours. The process time in the recovery section is preferably maintained at less than 1 hour and more preferably at less than xc2xd hour.
The reduction section is preferably a shredder-grinder type unit that takes the polystyrene-type material from an external source, such as expanded polystyrene foam (EPS foam) used in packaging crates for food shipment, protective packaging for shipment of electronic components, display materials, safety helmet liners, discarded disposable dinner utensils, and such, and reduces its size to allow exposure of the surface for greater contact with the solvent.
The waste polystyrene material, optionally size-reduced, is fed into a dissolve unit where the reusable polystyrene solvent is introduced. The dissolving of the polystyrene-type materials may be enhanced through use of heat and agitation of solution. Means to heat and agitate the solution in the dissolve unit are provided. Further, means to monitor and control the agitation, viscosity, pressure and temperature are also provided. Means to condense and return the solvent, which may volatilize during the dissolving process, to the solution is also provided. One or more of the filters of the filter section may be integral with the dissolve unit. Thus, the filters to remove any particulate matter from the solution may be internal or external to the dissolve unit. These filters will remove all solids present due to either external contamination or other materials such as plastics, paper, tape, etc., which are not dissolved by the solvent and will further clarify the solution to remove finer particulate matter. After passing through the dissolve unit and the filter section, the polystyrene solution is then transferred to the recovery section.
The main portion of the recovery section is preferably an extruder-type unit set up as a devolatilizer. Preferably, the recovery section also includes a preheater to superheat the solution prior to introduction into the extruder. The preheater includes means to monitor and control the temperature and pressure so that the desired maximum temperature for the material being processed (less than 190xc2x0 C.) is not exceeded. The preheated solution is then pumped under pressure through a valve into the extruder feed section, causing immediate flashing of the low boiling, high vaporization rate solvent that greatly increases the solids content fed to the extruder screw. Preferably, the system will be closed to prevent loss of solvent to the atmosphere and facilitate reuse of the solvent in the process. Thus, except for the introduction of the waste polystyrene material into the reduction unit, the system, including introduction of the waste polystyrene material into the dissolve unit to its exit from the extruder plus the internal recycling of the reusable polystyrene solvent, preferably remains closed to the atmosphere. Maintaining the system closed to the atmosphere also increases the environmental safety of the process.
The extruder unit, including the feed section, of the recovery section preferably will include several ports to transfer the volatilized solvent to a condensation unit for capture of the devolatilized solvent. Preferably, one or more ports at the end of the extruder will be attached to vacuum means to enhance complete devolatilization of the extrudate melt. The continual introduction of mechanical work and addition of some external heat in the extruder along with application of vacuum ensures that the remainder of the solvent is volatilized. Means to monitor and control the temperature and pressure are provided in the extruder portion of the recovery section in order to facilitate devolatilization, extrusion and maintaining the temperature below 190xc2x0 C., preferably below 180xc2x0 C. After the exit from the end of the extruder, the polystyrene-type polymer melt is cooled and then preferably chopped into pellets in a manner known in the art. The reclaimed polystyrene material exiting from the extruder may be configured in larger pieces as required for its use in the end manufacturing process in a manner known in the art. The low boiling point and high evaporation rate of the solvent allows devolatilization to occur at a lower temperature than is standard for production of polystyrene-type polymeric materials, that is below 190xc2x0 C. This lower processing temperature (and a shorter processing time) substantially eliminates degradation of the waste polystyrene material introduced into the system. Thus, the reclaimed polystyrene-type material product will be within the specification ranges of the input polystyrene-type material for reuse to manufacture like grade polystyrene products.
The condensation of the solvent for reuse in the process of the invention is accomplished by passing the vapors through a condenser that is connected to chillers to ensure the reduction of the solvent vapor to below its boiling temperature and the reduction of its vapor pressure sufficiently to cause condensation. The condensed solvent is then transferred to interim storage for further reuse.